- by Steve LaCerra
in Theory and Practice
Last month (FRONT of HOUSE, Nov. 2016, page 41), we discussed the concept of word clock. Here’s the highlight reel:
• Almost every digital audio device has an internal word clock
• All devices sharing digital audio must be synchronized to the same clock or we’ll hear noise, distortion or glitches
• There can be only one word clock master device in a digital audio system. All other devices must follow or slave to that master clock.
• Word clock can be routed using a variety of different physical connections
• Word clock can be transmitted on the same port, at the same time with digital audio, or via a separate, dedicated connection (usually a BNC jack).
(To review the full text from last month, CLICK HERE).
The example we examined last month consisted of a digital console feeding its AES/EBU digital output to the AES/EBU digital input of a system processor. To avoid noise and dropouts, we set the console’s clock to internal and the processor’s clock to AES/EBU, meaning the processor will lock to and follow the console’s clock that’s sent along with the AES/EBU digital audio output. Everyone is happy.
Things become considerably more complicated when we increase the number of digital devices in the chain. Let’s suppose we plan to use the same digital console and processor, but we have two main vocalists in our band, both who want to use a “vocal channel” with a digital output (as discussed in the Oct. 2016 Theory & Practice — “Where’s Your Money?”— on page 57). We can start with the same routing of AES/EBU digital audio as in the previous example (console-to-processor) and there are no problems.
But what about the digital output from the vocal channels? The clocks in the vocal channels must be synchronized to the same clock signal as the console and processor. One possibility — if and only if the console has an AES/EBU digital input — is to make the vocal channel the master. Patch the AES/EBU output of the vocal channel to the AES/EBU input of the console. Set the vocal channel’s clock to internal, and then set the console and processor clocks to AES/EBU.
This configuration presents problems:
(1) Some digital consoles don’t have an AES/EBU input.
(2) We still have not figured out how to integrate the second vocal channel.
(3) Daisy-chaining may introduce some unexpected challenges (read on).
Many engineers will be tempted to say “Aw, just use the analog outputs from the vocal channels and patch them to analog inputs on the console.” And they’d have a valid point, because the can of worms we’re about to open is ugly.
What if you really want to use the digital output from those vocal channels, or need to interface a DAW with your console for track playback or recording purposes?
If you plan to stay entirely within the digital domain, it’s a fact of life that, at some point, you’re going to need to route the same word clock to multiple destinations.
So how do you do it? Well, we start with the fact that splitting a word clock signal using a “Y” cable will damage the signal, so home-brew AES/EBU or coax “Y” cables are not an option [Note: coaxial cables are typically used to patch word clock via the BNC word out/in jacks found on a lot of pro audio gear]. Next, we have to consider the fact that word clock deteriorates when it is daisy-chained or passed through one device to the next to the next.
Back in the days when I was using digital tape machines in my studio, this was a big issue because there were so many devices in need of a word clock signal, including the tape machines, a digital console, DAW interfaces, mic preamps with A/D conversion, and so on. Daisy-chaining word out from device #1 to word in of device #2, then through to device #3 and so forth, was a disaster (e.g., clicks, pops, and gross distortion) — especially when the cable runs became longer than a few feet.
We need a direct connection from the word clock master and the word clock input for each and every device in the system. Then it becomes like soldiers marching completely in sync, every soldier directly following the master for timing.
The Master Clock Solution
This is where the concept of a master clock device enters. A master clock is a box that usually does nothing but generate a strong, stable, accurate word clock signal, and it provides multiple output jacks to distribute that clock signal to various devices.
Take a look at Fig. 1. Here’s the rear panel of an Antelope Audio LiveClock, a portable, dedicated word clock generator. You’ll see four BNC word clock outs, two AES/EBU outs, and two S/PDIF outs.
We’ve already mentioned that the BNC jacks carry word clock only, but the same is true of these particular AES/EBU and S/PDIF ports: They are not carrying audio, only word clock. When the LiveClock is used as a master clock, you can “home run” cables from it to a variety of devices over a variety of protocols, without signal degradation. In most cases, the LiveClock would be set to Internal, establishing it as the master clock for the entire system. The other devices would be set to receive clock via their BNC word in, or AES/EBU in or S/PDIF in jacks.
You’ll notice that LiveClock also has jacks labeled “10M In” and “WC In.” These connectors are there in case the LiveClock needs to slave to a Mother Of All Master Clock Devices and then output a “refreshed” clock signal to additional slaves.
There’s a second (and probably more popular) reason why some engineers use external master clocks, and this idea opens an even uglier can of worms:
Using a master clock as a means of improving the audio quality of existing digital gear. I’m not taking sides here, but there are folks who feel that a higher quality master clock will produce higher quality audio due to the reduction of jitter (minute variations in clock speed). And, in fact, many sound companies have extended the lifespan of their older digital desks by using an external master clock to improve the sound quality of that console. The other camp feels this is a bunch of snake oil.
Unfortunately, I can’t tell you with any certainty whether or not adding a high quality, external clock will improve the audio quality of your system, though it will certainly solve the synchronization issues. I can say that there may be some truth to the fact that older devices with A/D converters — A/D conversion being the sore spot where audio problems manifest due to poor clocking — may benefit from a more stable and accurate word clock signal. It would appear that the higher up the food chain we go, the less difference that an external clock generator will produce in the overall sound of the system. All things being equal — and they never are — more expensive digital devices probably have more engineering resources dedicated to their internal clock generators than cheap digital gear.
The bottom line? Don’t listen to me, listen to your ears.
Steve “Woody” La Cerra is the tour manager and front of house engineer for Blue Öyster Cult.